1. Field of the Invention
The present invention relates to semiconductor die or semiconductor devices mounted on substrates. More specifically, the present invention relates to a method and apparatus for underfilling the gap between a bumped or raised semiconductor die or semiconductor device and a substrate.
2. State of the Art
Semiconductor die and bumped die technology is well known in the art. A semiconductor die or bumped (raised) die includes a bare or unpackaged semiconductor die having bumps on the bond pads formed on the active surface or front side thereof. The bumps located on the bond pads of the semiconductor die are used as both electrical connectors and mechanical connectors to attach the semiconductor die to a substrate. The semiconductor die is flipped and bonded to a substrate via the bumps located on the bond pads of the semiconductor die. Several materials are typically used to form the bumps on the semiconductor die, such as solder, conductive polymers, etc. Typically, if the bumps located on the bond pads of the semiconductor die are solder bumps, the solder bumps are reflowed to form a solder joint between the semiconductor die and the substrate, the solder joint forming both electrical and mechanical connections between the semiconductor die and the substrate. In any event, due to the presence of the bumps on the semiconductor die, a gap is formed between the substrate and the active surface of the semiconductor die attached thereto. Since the substrate is not planar and since the solder bumps are not of uniform size, the height of the gap between the semiconductor die and the substrate will vary.
Typically, since the semiconductor die and the substrate have different coefficients of thermal expansion, have different operating temperatures and have different mechanical properties with differing attendant reactions to mechanical loading and stresses, the individual joints formed by the bumps between the semiconductor die and substrate are subject to different levels of loads thereby having different stress levels therein. Therefore, the bumps must be sufficiently robust to withstand such varying loads and stress levels to maintain the joint between the semiconductor die chip and the substrate for both electrical and mechanical connections therebetween. Additionally, the bumps must be sufficiently robust to withstand environmental attack thereto. To enhance the joint integrity formed by the bumps located between the semiconductor die and the substrate, an underfill material typically comprised of a suitable polymer is introduced in the gap between the semiconductor die and the substrate. The underfill material serves to distribute loads placed on the semiconductor die and substrate, transfers heat from the semiconductor die, provides a reduced corrosion environment between the substrate and semiconductor die and provides an additional mechanical bond between the semiconductor die and the substrate to help distribute loading and stress on the semiconductor die and bumps.
While the use of an underfill material between a semiconductor die and a substrate is recognized as an improvement from a reliability perspective, filling the gap between the semiconductor die and a substrate with underfill material poses problems from a manufacturing perspective. Among the problems is (1) the ability to uniformly fill the gap between the semiconductor die and the substrate with underfill material without voids and (2) the time required for filling the gap between the semiconductor die and the substrate with underfill material. In any event, if the gap between the semiconductor die and the substrate is not uniformly filled and voids occur therein, greater problems may occur than if no underfill material were used to fill the gap.
Currently, various methods are used to minimize the presence of any voids in the underfill material in the gap between the semiconductor die and a substrate. For example, one underfill method uses a one-sided or two-sided dispensing process where the underfill material is dispensed along only one side or two adjacent sides of the semiconductor die. The underfill material is allowed to freely flow and, with the action of capillary forces between the semiconductor die and substrate, pushing air existing in the gap between the die and the substrate from opposing sides of the semiconductor die as the underfill material fills the gap, thereby minimizing potential voids. Although this method is somewhat effective in minimizing voids in the underfill material in the gap between the semiconductor die and the substrate, the underfill method typically requires a relatively lengthy period of time for the underfill material to flow through the gap.
In an effort to decrease the period of time for the underfill process, U.S. Pat. No. 5,710,071 to Beddingfield et al. discloses a method of mounting a semiconductor die over an aperture in a substrate and dispensing the underfill material along the entire periphery of the semiconductor die. The underfill material flows through the gap between the semiconductor die and the substrate via capillary action toward the aperture in the substrate, thereby expelling air in the gap through the hole in the substrate to minimize voids in the underfill material.
Other methods for underfilling the gap between a semiconductor die and substrate to minimize voids in the underfill material include either injecting underfill material along one or two sides of the semiconductor die mounted on the substrate or injecting underfill material through an aperture centrally formed in the substrate below the semiconductor die, in each instance, the underfill material flowing by capillary action to fill the gap.
U.S. Pat. No. 5,766,982 to Akram et al., discloses a method of injecting underfill material along the sides of a semiconductor die mounted on a substrate and/or through an aperture in the substrate located below the semiconductor die mounted on a substrate utilizing capillary force to fill the gap between the semiconductor die and the substrate and further utilizing gravitational force to fill the gap by placing the substrate and semiconductor device on an inclined plane with or without a barrier at the lower side of the semiconductor die to prevent the underfill material from substantially flowing beyond the lower side of the semiconductor die.
Although such methods for filling the gap between a semiconductor die and a substrate with underfill material may be satisfactory, it is desirable to reduce the length of the period of time required for filling the gap. Therefore, it would be advantageous to develop a satisfactory method for filling the gap between a semiconductor die and a substrate with underfill material requiring a minimum length of time.